Role of endogenous melatoninergic system in development of hyperalgesia and tolerance induced by chronic morphine administration in rats.

Morphine is a widely used analgesic for various types of pain. However, its efficacy is impeded by development of hyperalgesia and tolerance. Melatonin has antinociceptive effect and is involved in morphine-induced hyperalgesia and tolerance but the mechanism of its involvement remains to be defined. In this study, we established a rat model of morphine-induced hyperalgesia and tolerance. We determined the serum level of melatonin and expression of μ-opioid receptor (MOR), melatonin receptor (MT1, MT2) and protein kinase C γ(PKCγ) in the spinal dorsal horn of the rats with morphine-induced hyperalgesia and tolerance. Comparing with control group (n=6), the group (n=6) of rats with morphine-induced hyperalgesia and tolerance exhibited a significant lower serum melatonin level, reduction in expression of the MT1, but up-regulation of the PKCγ in the spinal dorsal horn. These results may facilitate revealing the mechanism of opioid-induced hyperalgesia and tolerance and exploring new therapeutic remedy for pain management.